It is proposed to study the control of folate metabolism in developing and adult rat liver, and the changes occurring in regenerating liver and hepatomas. We will examine the isozymes of dihydrofolate reductase and determine the electrophoretic patterns in hepatomas. We will also investigate the balance between different routes of utilization of tetrahydrofolate cofactors, and study properties of the seven enzymes involved. Folate cofactor levels, one-carbon-group donor concentrations, NADP+ and NADPH will be measured in freeze-clamped liver and hepatomas. The possible modification of physiological control of folate enzymes in hepatomas will be studied. Principal sources of one-carbon groups in liver and hepatomas will be identified, and the distribution of one-carbon groups from various sources between different acceptors will be studied. The program of experimental chemotherapy will exploit the changes in hepatoma biochemistry. Effects of antimetabolites will be studied on reconstituted multienzyme systems, on hepatomas growing in culture, and as solid tumors. Mathematical models will predict optimal conditions for chemotherapy; by using the knowledge of hepatoma folate biochemistry obtained in this study, drug toxicity to hepatomas may be maximized whilst toxicity to host tissues is minimized. Drug combinations predicted biochemically to give high therapeutic indices will receive detailed pharmacological evaluation.